Measurement while drilling (MWD) and logging while drilling (LWD) tools are commonly used in oilfield drilling applications to measure physical properties of a subterranean borehole and the geological formations through which it penetrates. Such M/LWD techniques include, for example, natural gamma ray, spectral density, neutron density, inductive and galvanic resistivity, acoustic velocity, acoustic caliper, downhole pressure, and the like. Formations having recoverable hydrocarbons typically include certain well-known physical properties, for example, resistivity, porosity (density), and acoustic velocity values in a certain range.
In some drilling applications it is desirable to determine the azimuthal variation of particular formation and/or borehole properties (i.e., the extent to which such properties vary about the circumference of the borehole). Such information may be utilized, for example, to locate faults and dips that may occur in the various layers that make up the strata. In geo-steering applications, such “imaging” measurements are utilized to make steering decisions for subsequent drilling of the borehole. In order to make correct steering decisions, information about the strata is generally required. As described above, such information may possibly be obtained from azimuthally sensitive measurements of the formation properties.
Azimuthal imaging measurements typically make use of the rotation of the drill string (and therefore the LWD sensors) in the borehole during drilling. Conventional flux gate magnetometers are utilized to determine the magnetic toolface angle of the LWD sensor (which, as described in more detail below, is often referred to in the art as sensor azimuth) at the time a particular measurement or group of measurements are obtained by the sensor. However, conventional magnetometers have some characteristics that are not ideally suited to imaging applications. For example, flux gate magnetometers typically have a relatively limited bandwidth (e.g., about 5 Hz). Increasing the bandwidth requires increased power to increase the excitation frequency at which magnetic material is saturated and unsaturated. In LWD applications, electrical power is often supplied by batteries, making electrical power a somewhat scarce resource. For this reason, increasing the bandwidth of flux gate magnetometers beyond about 5 Hz is sometimes not practical in certain downhole applications. Moreover, conventional magnetometers are susceptible to magnetic interference from magnetic ores as well as from magnetic drill string components. For geo-steering applications, directional formation evaluation measurements are preferably made very low in the bottom hole assembly (BHA) as close to the drill bit as possible where high magnetic interference is known to exist. Magnetic interference from steering tool and mud motor components is known to interfere with magnetometer measurements.
Therefore, there exists a need for an improved sensor arrangement for making directional formation evaluation measurements. In particular, there is a need for a sensor arrangement suitable for making high frequency tool face angle measurements near the drill bit (e.g., in the body of a steering tool located just above the bit).